This invention relates to internal reflection spectroscopy; and its primary purpose is to provide an apparatus which permits relatively deep immersion of an internal reflectance accessory into a container whose contents are to be spectroscopically analyzed.
An accessory referred to as a "prism liquid cell" is marketed by Harrick Scientific Corporation. This accessory is used to replace "amalgamated sealed cells", which were previously used to hold sample material, through which radiation was passed to provide transmission spectroscopy.
In internal reflection spectroscopy by an accessory extending into the sample material, an internally reflecting element (IRE) is surrounded by the sample; and the analytical radiation is essentially confined inside the IRE. Infrared light enters and leaves the IRE from one end, which in the Harrick accessory has the shape of a 90.degree. rooftop. The Harrick accessory is a rectangular cross-section crystal (typically composed of zinc selenide) having its outer (non-immersed) end cut at an angle of 45.degree. to each of its long sides.
Infrared light from a source enters the first inclined side of the IRE rooftop, generally on a perpendicular path. The infrared light is reflected first from one wall of the IRE, then from the opposite wall of the IRE, and so on, until it reaches, and is reflected by, the end of the IRE. It is then returned, along a path parallel to the incoming radiation, by reflection back and forth across the IRE until it exits the IRE from the second inclined side of the rooftop. And it is then directed to an infrared detector.
If the IRE (also referred to as a prism or crystal) is surrounded by air, the internal radiation from the source will be totally reflected. However, if the IRE is in contact with an infrared absorbing material, such as a liquid chemical, the radiation will be selectively absorbed at various wavelengths, resulting in an infrared spectrum. The amount of radiation absorbed is influenced by the angle of incidence of the radiation on the sides of the IRE.
IREs are often referred to as attenuated total reflectance (ATR) crystals, because the internal reflectance permits a limited amount of light absorption by the sample surrounding the crystal. The angle of incidence of the internal light on the crystal walls must be at or above the critical angle. If the angle of incidence is too small, excessive radiation will leave the crystal and be absorbed by the sample. If the angle of incidence is too large, excessive radiation will be internally reflected, and insufficient radiation will be absorbed by the sample to provide adequate analytical information. In other words, sample absorption of radiation is necessary, but sufficient radiation must return and exit from the crystal on its way to the detector.
There is a major need for a sample analyzing spectroscopic accessory which can extend deeply inside a container. Such an accessory would be invaluable in providing information during processing of the contents of the container. It would also permit ready evaluation of the condition of previously stored materials.
In situations where higher frequency radiation is useful (e.g., in the visible range), fiber optic light transmission may be used to get the radiation into and out of the deeply immersed IRE. However, the use of fiber optic transmission for infrared radiation is prohibitively expensive.